1. Field of the Invention
The present invention relates to an LED driver. More particularly, the present invention relates to an LED driver, which drives light emitting diodes at low power to improve efficiency.
2. Description of the Related Art
Light emitting diodes (LEDs) are used as a light source of a liquid crystal display (LCD) apparatus, as well as a digital micromirror device (DMD) display apparatus such as a digital light processing (DLP) projection TV, a projector, and the like, using a DMD.
FIG. 1 illustrates the DMD display apparatus which employs LEDs as the light source. The DMD display apparatus employs a plurality of LED modules 210 corresponding to respective colors of red, green and blue (RGB).
The LED modules 210 are driven by an LED driver 200, and the driven LED modules 210 emit red green and blue light signals to sequentially project to the light onto a DMD module 230 through a lens 220. In a DMD, hundreds of thousands or even millions of mirrors 240 are integrated into the DMD module 230 by a micro electro-mechanical system (MEMS) process, and independently turn on and off. Accordingly, RGB color signals projected to the DMD module 230 display a predetermined picture on a screen 250.
A DMD display apparatus using LEDs as the light source has several advantages as compared with a conventional display apparatus using a discharging lamp as the light source. A DMD display apparatus has high light efficiency, a longer life span of the LEDs than the discharging lamp, and does not require a mechanical apparatus such as a color wheel.
The LED driver 200 for driving the LED modules 210 typically comprises a circuit configuration as shown in FIG. 2. The LED driver 200 in FIG. 2 may be referred to as a switch mode driving circuit. The LED driver 200 comprises a current detector, an error amplifier 272, a PWM modulator 274, a gate circuit 276, a switch 278, an inductor 280, a first diode 282, a second diode 284 and a switch block 286.
The LED driver 200 detects the current flowing in the LED modules 210 through the current detector, compares a voltage corresponding to the detected current to a target voltage Vref through the error amplifier 272, and outputs a voltage difference signal.
The PWM modulator 274 compares an output of the error amplifier 272 and a predetermined triangular wave, and generates a pulse width modulation (PWM) signal. The gate circuit 276 drives the switch 278 which is realizable as a metal-oxide semiconductor field effect transistor (MOSFET) by the pulse width modulation (PWM) signal. The inductor 280 integrates a square wave pulse output and allows the LED modules 210 to be supplied with a direct current having a ripple corresponding to the switching of switch 278.
As the amount of light for each of RGB colors is different in white light, the amount of current Io flowing in the LED modules 210 should be different for each of RGB colors, and it can be adjusted through the reference voltage Vref. The switch block 286 comprises a divergence switch which is connected to the LED module 210 corresponding to each of the RGB colors, and makes the current Io flow in the LED module 210 by synchronizing the switches 286 with changes of the reference voltage Vref. Thus, as Vref changes, the switches Vr, Vg and Vb turn on/off so that different levels of current flow through the LED's according to color.
The LED module 210 which is driven by the LED driver 200 comprises a single module that connects dozens of LEDs in serial/parallel corresponding to each of RGB colors. A current of more than 20 A and a voltage of more than 20V is typically used to drive the LED module 210. Also, the smaller the ripple of current Io is, the better it is for equalizing the characteristic of the picture quality. Also, the switching and the transient phenomenon should be fast for high light efficiency when sequentially driving the LED module 210 corresponding to each of RGB colors.
The driving circuit of the switch mode in FIG. 2 is enough to ensure high efficiency with respect to high power. However, because an inductor is used to reduce the ripple current, the switching frequency must be raised or the inductor must have a large inductance. As the inductance is raised, the transient phenomenon becomes slow, thereby lowering the light efficiency.
If driving the LED driver 200 with a discontinuous current mode (DCM) as illustrated in the pair of waveforms shown in the upper part in FIG. 3, a dead zone is lengthened in which the DMD can not operate due to the slow transient phenomenon of an inductor having a large inductance, thereby lowering the light efficiency.
As shown in a lower part of FIG. 3 as a pair of wave forms, the flow of the current Io is in a continuous current mode (CCM) and the light efficiency is increased a little if the dead zone is reduced while changing the divergence switch in the LED driver 200. However, a reverse recovery current is generated in the second diode 284 while changing the divergence switch, which may adversely affect the stability of the circuit and generate unwanted electromagnetic interference (EMI).
For example, the reverse recovery current generated while a current of about 20 A flows in the LED module 210 may be a few tens or hundreds of amps. As the reverse recovery current flows through the LED module 210, deterioration of the LEDs is accelerated. Also, the DMD module is turned off until the reverse recovery current disappears and the circuit is stabilized, thereby lowering the light efficiency.
Accordingly, there is a need to raise the light efficiency by minimizing the dead zone in the divergence switch without causing a reverse recovering current, and improving a transient response through increase of charging and discharging slopes of the current Io. The charging and discharging slopes of the current Io of the LED driver 200 are provided as Equations 1 and 2:
                                          ⅆ            i                                ⅆ            t                          =                              Vcc            -                          V              D                                L                                    [                  Equation          ⁢                                          ⁢          1                ]                                                      ⅆ            i                                ⅆ            t                          =                              V            D                    L                                    [                  Equation          ⁢                                          ⁢          2                ]            where, Vcc refers to a power voltage, VD is a voltage applied to a diode, and L is the inductance of the inductor 280. Because VD is invariable, the discharging slope given in equation 2 is also invariable corresponding to a predetermined inductance. Meanwhile, the charging slope is variable as the Vcc can be changed.
The charging and discharging slopes of the current should be as large as possible to obtain a fast transient response for high light efficiency. As the discharging slope is fixed, the charging slope may be raised for a fast transient response. However, in order to adequately raise the charging slope, the power voltage Vcc may be too large. For example, the power voltage Vcc should be doubled relative to the LED voltage VD as Vcc=2VD to make the charging slope and the discharging slope identical. However, as the power voltage becomes large, accordingly a switching noise and the EMI becomes large and components having a of the ability to withstand the higher voltage are needed, thereby raising the production costs.